Calibrating Load Cell Transmitter for 0-5V Output

Hey everyone, I'm having trouble calibrating my load cells with a transmitter from ATO. I'm trying to get a 0-5V output, but the readings are inconsistent. Every time I recalibrate, the values seem way off.

I’m using 500kg S-type load cells and calibrating with a 24kg weight (SKU: ATO-LCTR-OA). Initially, I followed the calibration process described in the manual, but I noticed that the load cell was giving a negative power value when the transmitter was set to 0V. So, I recalibrated to make sure the load cell gave a positive signal to the transmitter. That seemed to help with accuracy, but now my zero point is at 2V instead of 0V.

Any ideas on how to fix this? Thank you very much!

Emma wrote:Emma, for a 500kg load cell, we recommend using at least 50kg for calibration instead of 24kg.
If the transmitter is displaying a negative signal, it might be due to reversed polarity. Try swapping the green and white signal lines and recalibrating.

ATO wrote:Ok, I already swapped the green and white wires (now set as green, white, red, and black), and while this gives a positive reading, the main issue remains. When measuring the signal directly from the load cell (S- and S+), I see that if the signal is negative, it affects accuracy.

The weird thing is, after recalibrating so that S- and S+ show a positive value, my accuracy improves, but my zero point is stuck at 2V. Any thoughts?

Emma wrote:Emma, it sounds like your zero offset is incorrect. You should first ensure that your load cell outputs a small but stable voltage when no weight is applied. Then, before doing the span calibration (with weight), adjust the zero potentiometer on your transmitter to bring the output down to 0V. If your transmitter isn’t allowing you to adjust below 2V, there may be a range setting or an internal offset issue.

Emma wrote:You might want to check if your load cell excitation voltage is stable. If the excitation voltage fluctuates, it could affect your zero and span calibration. Also, are you using a proper differential measurement when checking S- and S+? If you're measuring relative to ground instead of between S- and S+, that could explain why you're seeing unexpected behavior.

Emma wrote:To clarify, calibration should be done in two steps:
Zero Calibration: With no load, adjust the zero potentiometer until the output is 0V.
Span Calibration: With a known weight (preferably 10% of full scale, so 50kg in your case), adjust the span potentiometer until the output corresponds to the expected voltage (e.g., 0.5V for 10% of 5V).
If your zero level remains at 2V, double-check the wiring and confirm that the output from the load cell itself is within expected limits before entering the transmitter. A video of the setup might help diagnose the issue further.

Thanks for all the suggestions! I’ll try power cycling and checking the excitation voltage stability. I’ll also redo the calibration using a 50kg weight instead of 24kg and see if that helps!